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Paris, November 18, 2008

Sea levels are still rising but for different reasons

Researchers at the Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (1) (CNRS/Université Toulouse 3/CNES/IRD) and at a subsidiary of CNES (CLS) (2) have discovered that the accelerated melting of continental icepacks is the major reason for the rise in sea level over the 2003 to 2008 period, something which has minimized the effect of thermal expansion of seawater. This question was resolved thanks to data from the French-American Satellite Jason-1, from two satellites of the GRACE space gravimetry mission and from the buoys of the Argo system. These results have been published online on the website of the journal Global and Planetary Change.

Between 1993 and 2003, the global mean sea level, measured very accurately by the French-American Topex/Poséidon satellites and their successor Jason-1, showed a relatively constant progression of 3 mm/yr. The last GIEC report, published in 2007, showed that more than half of this rise (approximately 1.5 mm/yr) was due to sea water expansion as it warmed up (steric contribution), while 1.2 mm/yr resulted from the reduction in mass of polar ice sheets and mountain glaciers. Since 2003 however, the situation has changed; a quite rapid rise (2.5 mm/yr) in sea water levels is still observed but, over the same period, the warming of the oceans is showing a plateau, only accounting for a rise of 0.4 mm/yr.

Thermal expansion was calculated using two independent methods:- The Argo network of buoys transmits water temperature and salinity profiles across all of the world's oceans. Since 2003, the analysis of all relevant data in the topmost 900 meters of sea water resulted in a steric contribution of about 0.4 mm/yr. - This value was independently confirmed by measurements from space by calculating the difference between the water level observed by the altimeters on Topex/Poséidon and Jason-1 and the increase in the ocean volume as witnessed by GRACE. The satellites indicate a steric contribution of 0.3 mm/yr, which is very similar to the value from the Argo buoys.

Consequently, it is above all the increase in the mass of sea water rather than its heat content that is behind the rise in sea level that has been observed since 2003. The increase in the mass of the oceans is equivalent to a rise of 1.9 mm/yr of the mean sea level. What is the source of this extra water in the oceans? Melting continental ice sheets. Data from GRACE has made it possible to measure changes in the mass of the two polar ice sheets in Antarctica and Greenland. These were responsible for a 1 mm/yr increase in sea level (i.e. twice as much as in the previous decade). For mountain glaciers, the most recent estimates from glaciologists show a contribution of 1.1 mm/yr (also higher than during previous years).

Thus, losses from glacial masses can easily account for why the mass of sea water is increasing and are responsible for 80 % of the average rise in sea level in recent years. Given the accelerated melting of glaciers and polar ice sheets, if the steric contribution returned to the values of the 1990s, a rise in sea level of around 4 mm/yr could not be excluded.